Like elsewhere in many coastal water, a bimodal pattern of plankton distribution in Sundarban estuaries is observed in Sundarbans as well. It means dense plankton population once in premonsoon (May) and again in postmonsoon (November) .
Dominant phytoplankton species in Sundarbans during are Ditylum, Ceratium, Biddulphia, Chaetoceros, Coscinodiscus, Thalassiothrix, Rhizosolenia and Thalassionema. In post-monsoon phytoplankton species of Bacteriastrum, Biddulphia, Protoperidinium were most dominant. During monsoon, dominant species are Skeletonema, Fragillaria, blue green and euglenoids.
Zooplankton variation is insignificant across the Sundarban estuary unlike phytoplankton. The eastern part of the Sundarban estuary (India) is dominated by phytoplankton species like Biddulphia diatoms and green and blue green algae, while the central part is dominated by a variety of diatom species viz, Chaetoceros, Coscinodiscus, Bacterioastrum, Cyclotella, Ditylum, Skeletonema, Thallassiothrix, Thalassionema and Triceratium. In contrary, the western region is dominantly represented by species of Fragillaria, Gyrosigma, Nitzschlia and Bacillaria.
Plankton assemblage and distribution however is affected by both biotic factors and abiotic factors which includes pH, temperature, salinity etc. Since temperature variation is low in this tropical climate of Sundarbans, salinity and nutrient are considered as the most important biological factors governing phytoplankton composition and abundance. In fact, phytoplankton and the biotic and abiotic factors (pH, alkalinity, and salinity) of the estuary are positively correlated i.e. rise of the factors will encourage a phytoplankton bloom.